Method for making structures having low radar reflectivity

ABSTRACT

A lightweight, three dimensional structural article having low radar reflectivity characteristics and good mechanical properties is fabricated from a syntactic foam core having a structural outer skin. A syntactic foam sealing adhesive film is applied to the surface of syntactic foam core and under the outer skin to both seal the foam core and adhere the outer skin. The syntactic foam sealing adhesive is formulated to provide a uniform layer and to reduce the fabrication time and labor while improving the overall radar absorbing characteristics of the structural article.

This application is a division of Ser. No. 08/947,223, filed Oct. 8,1997.

FIELD OF THE INVENTION

This invention relates generally to articles made from low-densitystructural materials and more particularly, to a method of fabricating alow density structural article having a low dielectric constant.

BACKGROUND OF THE INVENTION

A number of aerospace and other structural applications require the useof non-metallic materials which are lightweight and have a low densityalong with good mechanical and structural properties. More recently,there is an increased need for such materials which also act aselectromagnetic and particularly, radar absorbing or transparentmaterials. For example, radomes that house radar antennas must belightweight, yet have sufficient structural strength and toughness towithstand the aerodynamic forces experienced during flight. Thematerials used to construct the radome must be of a sufficiently lowdielectric so as not to interfere with the transmitted signals.

Syntactic foams are often used as a lightweight material having goodmechanical and structural properties as well as radar absorbingcharacteristics. These syntactic foams include open pores or cells thatare formed by incorporating small diameter hollow objects ormicroballoons into a resin bonding agent which is then cured into a foamstructure. Syntactic foams are generally structurally superior to blownfoams, such as polyurethane foam and expanded polystyrene foam. Inaddition, the lower the density of the foam, the lower the dielectric.However, these syntactic foams generally do not have sufficientmechanical properties, such as strength and outer surface durability,required for aerospace and related structural applications.

In an effort to improve the mechanical and other properties of thesesyntactic foams, an outer layer of a structural material such as anouter skin is often applied and bonded to the syntactic foam article orcore. This outer skin generally improves the mechanical properties ofthe foam core adding structural strength and rigidity to the finishedstructural article. However, bonding or otherwise attaching an outerskin to the syntactic foam core has a number of drawbacks and problems.

Typically, a resin adhesive is used to secure the outer skin to thesyntactic foam core. Adhesives are also used to secure articles madefrom the syntactic foam core to other structures. The adhesive isapplied to the desired surface of the foam core using a variety ofmethods and techniques, all of which are labor intensive and timeconsuming. In addition, the adhesive typically has poor electricalproperties relative to the syntactic foam. Specifically, applying anadhesive to the syntactic foam can significantly raise the dielectric ordegrades the overall electrical properties, such as the radar absorbingcharacteristics of the finished structural article.

Prior to applying an adhesive, the syntactic foam core must be sealed.Specifically, any surface on the foam to which a resin type adhesive, abonding agent or most any other substance will be applied must, bepre-sealed. Pre-sealing prevents the leaching or other draining of theresin matrix from the adhesive or other applied substance into the cellsor open pores of the foam core. However, pre-sealing the syntactic foamcore is also labor intensive, time consuming, expensive, and generallydegrades the radar absorbing characteristics of the finished structuralarticle.

One method of sealing a porous foam, such as a blown foam, is describedin U.S. Pat. Ser. No. 5,472,541, issued to Simmons et al. This patentdiscloses a method for applying a thin coating of a powdered adhesive toa porous foam article to form a sealed layer. After being coated, thefoam article is placed on an adhesive coated belt. The belt is thenmoved into a heating zone where the coating of the adhesive is fullymelted to the lower surface of the foam article.

Methods of sealing and applying an adhesive to a surface of a syntacticfoam article or core include coating the desired surface of thesyntactic foam article with a mastic like sealer. This sealer isgenerally applied manually, by hand application. After the sealer hasbeen applied, the adhesive can be applied on top of the sealer. Thesemethods, including the applied materials, have many disadvantages. Forexample, the currently used sealing and adhesive materials havesignificantly higher dielectric constants than the syntactic foam core.This degrades the radar absorbing characteristics of the overallstructural article. The application of both the sealer and the adhesiveis time consuming, expensive and the rigors of the manual applicationprocess has the potential to crush or otherwise damage the syntacticfoam core. It is also difficult to control the thickness of the sealerand adhesive using the current materials and methods.

There is thus a need for a structural article having a syntactic foamcore with a structural outer skin and which has improved radar absorbingcharacteristics. There is also a need for a method of fabricating athree dimensional structural article having improved radar absorbingcharacteristics.

SUMMARY

The present invention overcomes these problems of the past by providinga lightweight three dimensional structural article, made from aplurality of materials, and which has excellent mechanical propertiessuch as strength and durability as well as good radar absorbingcharacteristics. The structural article is made from a structural foamcore having a structural laminate outer skin and can be formed into mostany shape or configuration. In addition, the materials and particularly,the resin systems of the various materials may be specificallyformulated and adapted to provide a finished structural article having adesired density and having desired radar absorbing characteristics. Byusing materials specifically formulated for their improved radarabsorbing characteristics and mechanical properties, the finishedarticle retains overall low radar reflectivity characteristics.

The present invention also satisfies the need for a method offabricating such a lightweight three dimensional article. The methodutilizes a syntactic sealing adhesive foam which is compatible with thestructural outer skin and which seals the structural foam core. Thesyntactic foam sealing adhesive material, has a low dielectric constantfor improved radar absorbing characteristics. In addition, the materialscomprising the syntactic foam sealing film can be formulated to varydensity and dielectric constant and thus, modify the properties of thefinished structural article. The sealing adhesive film allows the outerskin to be applied to the structural foam core and cured to form thefinal structural article.

The present invention is generally directed to a three dimensionalstructural article having a desired configuration and improved radarabsorbing characteristics. The article is made from a plurality of lowdensity materials, each having a low dielectric constant. Particularly,the structural article comprises a laminate having a three dimensionalsyntactic foam core, a layer of a syntactic sealing adhesive film and astructural outer skin.

The syntactic foam core is formed substantially into the desired shapeor configuration and includes at least one exposed surface. Thesyntactic foam core, and particularly, the exposed surface is formedslightly smaller than the final desired configuration to accommodate thethickness of the sealing adhesive film and the outer skin. The outerskin covers the exposed surface and is sized to the desiredconfiguration. The outer skin provides a structurally strong articlewith a durable surface.

A layer of the foam sealing adhesive film is disposed between theexposed surface on the syntactic foam core and the outer skin. Thesealing adhesive film is preferably a prefabricated sheet of a syntacticfoam sealing adhesive film having a plurality of microballoonsintermixed with a resin. The prefabricated sheet is “B” stage cured andcut or otherwise sized to fit over the exposed surface. The sealing foamseals the exposed surface on the foam core and adheres the foam core tothe outer skin.

Methods for making a three dimensional article having a desired shape orconfiguration from a plurality of materials, each having a low densityand low radar reflectivity characteristics, for example, the presentlydescribed three dimensional article is included within the scope of thepresent invention. The method includes the steps of providing astructural thermoplastic syntactic foam core. The syntactic foam core isprovided or otherwise formed into substantially the desiredconfiguration and includes at least one exposed surface. Particularly,the foam core is formed slightly undersized from the desiredconfiguration to accommodate the thickness of the additional materialsrequired to fabricate the article.

An outer skin is also provided. The outer skin comprises a plurality ofstructural fibers intermixed with a resin matrix. The outer skin isformed or applied such that it conforms to the exposed surface on thefoam core.

The method also includes the steps of providing a layer of a syntacticfoam sealing adhesive film. The sealing adhesive film has a dielectricconstant of less than 2.5 to promote the low radar reflectivitycharacteristics of the finished article. The sealing adhesive film isadapted for sealing the syntactic foam core and for adhering the foamcore with the provided outer skin.

The formed foam core is supported such that the exposed surface is notinterfered with or obstructed. A layer of the sealing adhesive film isplaced over the exposed surface. The outer skin is pre-cured and thenplaced on the foam core such that the layer of sealing adhesive film iscompressed and sandwiched between the outer skin and the exposedsurface. This forms the desired configuration of the structural article,but in a pre-cured state. The pre-cured article is then secondarilybonded to form the three dimensional structural article. This step ofsecondarily bonding cures the sealing adhesive film and the pre-curedouter skin such that the outer skin is cured into a structural outerskin and is also secured to the foam core.

In another aspect of the present invention, the step of providing anouter skin comprises providing a layer of an uncured prepreg material.The uncured or partially cured prepreg outer skin is placed over thesealing adhesive film which was previously placed over the exposedsurface of the foam core. In this aspect of the present invention, theprepreg outer skin and the sealing adhesive film are co-cured.

This invention, together with the additional features and advantagesthereof, which is only summarized in the foregoing passages, will becomemore apparent to those of skill in the art upon reading the descriptionof the preferred embodiments, which follows in the specification, takentogether with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a exploded perspective view of an article according to theprinciples of the present invention; and

FIG. 2 is a cross-sectional view of an article according to theprinciples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference charactersdesignate identical or corresponding parts throughout the several viewsand embodiments, a structural, three dimensional article according tothe principles of the present invention is illustrated in FIGS. 1 and 2and designated by the reference numeral 10. As shown, the structuralarticle 10 is made from a plurality of materials which are securedtogether.

More specifically, the three dimensional structural article 10 of thepresent invention is a fabricated laminate which includes a low densitystructural foam core 12, a layer of a sealing adhesive 14 and an outerskin 16. The structural article 10 may be formed or otherwise configuredinto a desired shape or configuration, such as the configurationsillustrated in the figures. However, the desired configuration mayinclude most any three dimensional shape or configuration.

Each of the plurality of materials 12, 14 and 16 may comprise a lowdensity material which also acts as an electromagnetic radiationabsorber such that the fabricated structural article 10 has a lowdensity and low radar reflectivity characteristics as will be describedin greater detail below.

The structural foam core 12 may be a syntactic foam which has beenformed or otherwise shaped into the desired configuration. Preferably,the foam core 12 is a low density porous thermoplastic syntactic foamhaving unique electromagnetic energy absorption and structuralcharacteristics. The syntactic foam core 12 may comprise a polymer basedresin system 18 intermixed with a plurality of microballoons 20. Fibers22 may be incorporated into the resin system 18 to improve theabsorption of electromagnetic energy and more specifically, theabsorption and low reflectivity characteristics relative to radar waves.Preferably, the syntactic foam core 12 will have a dielectric constantof between approximately 1.0 and 2.0 and more preferably ofapproximately 1.2. A suitable syntactic foam is described in U.S. Pat.Ser. No. 5,532,295, issued to Harrison et al., and entitled,Thermoplastic Syntactic Foams and Their Preparation, and which is hereinexpressly incorporated by reference. The syntactic foam core 12 may alsobe molded as well as formulated to have different densities, mechanicalproperties and radar absorbing characteristics. The preparation of thefoam core and associated properties is also described in the Harrison etal. patent.

The resin system 18 may include a low molecular weight, partiallypolymerized oligomeric precursor to a polyethermide polymer. A suitableoligomeric precursor is currently sold under the trademark 1010P, byGeneral Electric Co. The resin 18 binds the microballoons 20 together ina spaced apart array. Air, as well the fibers 22 and other materials,may also be included within the interstices of the mixed resin 18 andmicroballoons 20 to vary the density as well as the mechanical andelectrical properties.

As mentioned, microballoons 20 are mixed and dispersed within the resin18 to create the syntactic foam core 12. The microballoons 20 may becommercially available hollow bodies ranging in diameter from about 350microns or less. The microballoons 20 are preferably present in amountsfrom approximately 3 percent to 90 percent by volume and morepreferably, from about 70 percent to 85 percent by volume. Preferredmicroballoons 20 are sold under the trademark Eccospheres SImicroballoons, by W. R. Grace & Co., Canton Mass.

As mentioned, fibers 22 may be incorporated into the resin system 18 toimprove the electrical as well as the mechanical properties of thestructural foam core 12. The fibers 22 may also be employed to reducethe bulk density of the mixed material to allow a lower densitysyntactic foam.

The fibers 22 must be compatible with the selected resin system 18 inorder to provide good coupling between the fibers 22 and the resin 18.Preferably, the fibers 22 are relatively short and electrically activeor resistive. The fibers 22 are typically of a diameter of less than 15micrometers and a length of no more than 800 micrometers. The fibers 22may be present in amounts from approximately 0.1 percent to 20 percentby volume.

The fibers 22, which may be resistive in nature, may absorb portions ofthe electromagnetic radiation and also re-radiate the absorbed energy toother neighboring fibers, resulting in additional energy loss. Thefibers 22 may convert some of the electromagnetic energy into heatthrough the resistance of the generated electrical current in thefibers. The electric field generated by the electromagnetic radiation,such as radar waves, is thus, decreased by both the retransmission ofthe energy and the conversion into heat. Additionally, the foam core 12may be formulated to provide good mechanical and structural properties,such as rigidity, low density and high strength.

The syntactic foam core 12 may be molded, formed or otherwise configuredinto substantially the desired configuration. Forming the foam core 12into the desired configuration includes forming or otherwise providingat least one exposed surface 24 to which the outer skin 16 may beapplied. Because the outer skin 16, as well as the layer of sealingadhesive 14 have a certain thickness, the foam core 12 is preferablyformed having a size and configuration slightly smaller than the desiredconfiguration of the finished three dimensional article. Particularly,the foam core 12 is sized such that the addition of the outer skin 16and the sealing adhesive 14 results in the finished structural article10 having the desired configuration.

The layer of sealing adhesive 14 may be applied to the exposed surface24 of the foam core 12. The sealing adhesive 14 acts to seal the porousexposed surface 24 of the syntactic foam core 12 and as an adhesive tosecure the outer skin 16. Preferably, the sealing adhesive 14 may be asyntactic foam sealing adhesive film having a dielectric constant ofless than 2.5. By using a syntactic sealing adhesive, having a lowdielectric, the electromagnetic radiation absorbing characteristics ofthe layer of sealing adhesive are improved. By using a layer of film 14,a simple application step is required to apply the sealing adhesive film14 to the foam core 12, greatly reducing the labor and time requirementsof prior methods and materials.

The syntactic foam sealing adhesive film 14 may be a resin based filmincluding a plurality of second microballoons 26 intermixed with asecond resin system 28. The second resin system 28 may be matched suchthat it is compatible with the resin system 18 of the structuralsyntactic foam core 12.

In a preferred embodiment, the sealing adhesive film 14 comprises alightweight resin base system 28, such as that sold under the trademarkHysol Syncore, by Cytec Engineered Materials of Havre de Grace, MD. Aplurality of second fibers or similar materials may also be incorporatedinto the second resin system 28 to modify the electrical and mechanicalproperties of the resulting sealing adhesive film 14, similar to thatdescribed for the structural foam core 12. Preferably, the sealingadhesive film has a dielectric constant of between approximately 1.4 and2.5 and has a film thickness of less than 0.03 inches.

The plurality of second microballoons 26 may preferably have a diameteror particle size range from approximately 5 to 300 microns and a balloondensity from approximately 0.18 g/ccm and 0.28 g/ccm. The secondmicroballoons 26 may also be similar or identical to the previouslydescribed first microballoons 20.

The sealing adhesive film 14 may be configured as a prefabricated sheetor film layer and may be adapted for sealing the exposed surface 24 ofthe porous foam core 12 and for adhering with the outer skin 16. Inorder to prevent the resin within the second resin system 28 fromleaching or otherwise draining into the open pores of the structuralfoam core 12, the prefabricated sheet of syntactic foam sealing adhesivefilm 14 may be slightly pre-cured. Preferably, the prefabricated sheetof sealing adhesive film 14 is “B” stage cured prior to its installationon the structural foam core 12. Small particles may also be added to theadhesive film 14 to reduce the tendency or characteristic to flow. Forexample, silica particles, such as particles sold under the trademarkCabosil, by Cabot Corporation, may be added to reduce the flowcharacteristics.

Preferably, the sealing adhesive film 14 is applied as a single sheet orlayer of film over the foam core 12. However, in large applications orwhere unique shapes are encountered, it may be beneficial or evenrequired to apply a number of different sections of the adhesive film 14to entirely cover the exposed surface 24. Joining the different sectionsof adhesive film 14 may be accomplished by overlapping or splicing.Preferably, butt splicing techniques may be used.

The outer skin 16 is applied over the exposed surface 24 of the foamcore 12 and against the sealing adhesive film 14. The outer skin 16 maybe adapted to cover at least a portion of the exposed surface 24 andpreferably mate with the entire exposed surface 24. Alternatively, theouter skin 16 may cover more than the exposed surface 24. The outer skin16 increases the rigidity and strength of the overall structural article10 as well as providing a durable outer surface. Once cured, theaddition of the outer skin 16 and the layer of sealing adhesive film 14to the foam core 12, forms the desired configuration of the structuralarticle 10. Preferably, the outer skin 16 has a thickness of less than0.1 inches and more preferably of less than 0.03 inches.

The outer skin 16 may comprise a cloth fabric or other structural fibersintermixed in a third resin system or matrix to form a thin structuralskin having a low dielectric. In a preferred embodiment, the outer skin16 may utilize a fabric or fibers having a very low dielectric losstangent as compared to fiberglass fabric. Such a fabric may include aquartz fabric supplied by J. P. Stevens Company. The third resin systemmay be a cyanate ester resin and is preferably a cyanate ester resinhaving a dielectric of less than 2.8 and more preferably of less than2.6. However, other fabrics and resins, including prepregs, whichprovide a structural skin and which are compatible with the foam sealingadhesive film 14 and the structural foam core 12 may also be used.

A method of fabricating a three dimensional structural article having adesired configuration from a plurality of materials is also includedwithin the scope of the present invention. In a preferred embodiment,the method is directed at fabricating the three dimensional structuralarticle 10 described above. Thus, the plurality of materials includes astructural foam core 12, an outer skin 16 and a sealing adhesive layer16. Preferably, the outer skin 16 and the sealing adhesive film 14 eachhas a minimized dielectric constant.

The method includes the step of providing a structural foam core base,such as the syntactic foam core 12 described above. The provided foamcore is molded or otherwise formed into substantially the desiredconfiguration and includes at least one exposed surface 24.Particularly, the structural foam core 12 is formed slightly undersizedfrom the desired configuration to accommodate the thickness of theadditional materials required to fabricate the overall structuralarticle 10.

The structural foam core 12 may be formed or otherwise configured intosubstantially the desired configuration using a number of methods. Onemethod includes using a mold or form which has been configured or shapedinto the desired configuration and also accommodates the thicknessrequirement of the layer of sealing adhesive 14 and the outer skin 16.The structural foam core 12 is cured within the mold to form the desiredconfiguration less the thickness and volume required for the layer ofthe sealing adhesive 14 and the outer skin 16. The general method ofusing a mold to form the syntactic foam core 12, including numeroususeable variations, is described in U.S. Pat. Ser. No. 5,532,295, whichis herein incorporated by reference as described above.

A second method for forming the syntactic foam core 12 intosubstantially the desired configuration includes forming the foam core12 into a first configuration. Portions of the foam core 12 may then beremoved until the desired configuration is formed. Additional materialmay be further removed from the exposed surface 24 to accommodate thethickness of the layer of sealing adhesive film 14 and the outer skin 16such that the overall structural article 10 comprises the desiredconfiguration.

The first configuration may include a billet or other section of foammaterial having any shape into which the foam core is originally molded.It is expected that these first configurations will typically includesquare billets or molded shapes which are slightly oversized and allowfor more accurate dimensioning through machining. Removing portions ofthe foam core 12 may be accomplished using any known machining andsimilar methods.

The method may also include forming the foam core 12 by joining a numberof individual foam core sections together to form substantially thedesired configuration. This may include joining a number of billets orfirst configurations of the foam material. Alternatively, the foam core12 may be made by joining a number of pre-shaped or configured sections.

The method includes placing a layer of a second syntactic foam sealingadhesive film 30 on a portion of a first foam core section 32.Preferably, the second sealing adhesive 30 is compatible with the firstsealing adhesive 14 and more preferably, the two are the same, althoughthis is not likely. A second foam core section 34 may then be placedagainst the first foam core section 32 to sandwich the layer of thesecond sealing adhesive film 30. The first and second foam core sections32 and 34 may be maintained together and heated at the required curingtemperature of the second sealing adhesive film 30. The curingpermanently joins the sections of the foam core material 32 and 34together while maintaining a electromagnetic radiation absorbingstructure having a low density and a low dielectric constant (preferablyless than 2.5 in regions adjacent the outer skin 14). The overall foamcore 12 made from the combined foam core sections 32 and 34 may then beformed as previously discussed.

The method also includes the steps of providing a layer of a syntacticfoam sealing adhesive film 14. As previously described, the sealingadhesive film 14 may include a prefabricated sheet of a syntactic foamsealing adhesive film 14 having a plurality of microballoons 26intermixed with a second resin 28.

Providing a layer of the syntactic foam sealing adhesive film 12 mayinclude providing the second resin matrix 28 and heating it to atemperature of between approximately 200 degrees and 275 degrees F. Thesecond plurality of microballoons 28 may be intermixed with the heatedresin 28 along with any other desired materials to form a mixture. Thismay include adding particles such as silica particles or Cabosil, aspreviously described. Mixing may be performed in a Ross type mixer whichmay also be used to degas the mixture. The mixture is then formed into athin film of between 0.005 inches up to approximately 0.03 inches.However, the thin film may also be formed into films of thickness up to0.1 inch or more. A doctor blade or other squeegee type device may beused to facilitate formation into the thin film or prefabricated sheet.For purposes of this disclosure, the term doctor blade shall include anyfilm fabricating device as is known to those of skill in the art offabricating films and thin layers of polymers and polymer likematerials. Preferably, the sealing adhesive film is then pre-cured to a“B” stage cure and cooled.

An outer skin 16 is also provided as previously discussed. The outerskin 16 is formed or applied such that it conforms to the exposedsurface 24 on the structural foam core 12. The outer skin 16 may beprovided in a pre-cured or structural form. In this embodiment, thestructural outer skin may be formed or otherwise configured into a shellhaving the desired shape of a portion of the structural article 10 andparticularly, the shape of the exposed surface 24. When using a cyanateester resin, the outer skin 16 may be pre-cured at a temperature ofapproximately 425 degrees F. to form a solid structural outer skin whichconforms to the exposed surface 24 of the structural foam core 12.However, a lower cure temperature may be preferred.

The outer skin 16 may also be provided as a prepreg material which hasnot been fully cured. In this embodiment, the prepreg outer skinmaterial 16 may be a layer or sheet of prepreg which is cut or otherwiseshaped to fit the shape and size of the exposed surface 24. The prepregouter skin 14 may be partially cured or “B” stage cured as previouslydescribed. The prepreg outer skin 16 is then placed over the layer ofsealing adhesive film 14. The layer of sealing adhesive film 14 and theprepreg outer skin may then be co-cured. This generally requires thatthe entire structural article 10 be heated to the curing temperature ofthe foam sealing adhesive film 14. It is expected that a temperature ofbetween approximately 300 and 400 degrees F. is preferred. However,differing temperatures may be used depending on the formulation of thefoam sealing adhesive film 14.

The formed structural foam core 12 is then supported such that theexposed surface 24 is not obstructed or otherwise interfered. A layer ofthe foam sealing adhesive film 14 is placed over the exposed surface 24.In general, the sealing adhesive film 14 may be cut to fit the exposedsurface 24 or alternatively, cut after being placed on the exposedsurface 24. The outer skin 16 may then be placed over the exposedsurface 24 and onto the foam core 12 such that the layer of sealingadhesive film 14 is compressed and sandwiched between the outer skin 16and the exposed surface 24. This forms the desired configuration of thestructural article 10, but in a pre-cured state.

The pre-cured article 10 is then cured to form the three dimensionalstructural article 10. This step of curing, which includes heating,cures the sealing adhesive film 14 such that the outer skin 16 issecured to the foam core 12. When the outer skin 16 is provided as aprepreg material which has yet to be fully cured, the curing cures bothof the sealing adhesive film 14 and the outer skin 16 together.Alternatively, when the outer skin 16 is pre-cured, only the sealingadhesive film 14 must be cured. Preferably, curing takes place within anautoclave or similar device.

It will be understood that various modifications can be made to thevarious embodiments of the present invention herein disclosed withoutdeparting from the spirit and scope thereof. For example, variouscombinations of the porous foam core, adhesive sealing layer film andthe outer skin are contemplated as well as various formulations of thesematerials. Also, various modifications may be made in the size,thickness, shape and configuration of the parts and their interaction.Therefore, the above description should not be construed as limiting theinvention, but merely as an exemplification of preferred embodimentsthereof. Those of skill in the art will envision other modificationswithin the scope and spirit of the present invention as defined by theclaims appended hereto.

What is claimed is:
 1. A method of fabricating a three dimensionalstructural article having a desired configuration from a plurality ofmaterials, each material having low radar reflectivity characteristics,said method comprising the steps of: providing a low density syntacticfoam core; forming said foam core substantially into the desiredconfiguration, said formed foam core having at least one exposedsurface; providing an outer skin having a plurality of structural fibersintermixed with a resin matrix, said outer skin being configured to fitover and mate with the exposed surface of said formed foam core andhaving a dielectric constant of less than about 2.8; providing asyntactic foam sealing adhesive film adapted for sealing said exposedsurface of said foam core and adhering with said outer skin, saidsyntactic foam sealing adhesive film having a dielectric constant ofless than about 2.5 and said foam core having a dielectric constantwhich is less than that of said syntactic foam sealing adhesive film;supporting the formed foam core; placing a layer of the sealing adhesivefilm on the exposed surface of the formed foam core; placing the outerskin on the formed foam core such that the layer of sealing adhesivefilm is sandwiched between the outer skin and the exposed surface; andcuring the structural article, such that the sealing adhesive filmsecures the outer skin to the foam core.
 2. The method as recited inclaim 1 wherein the step of providing a low density syntactic foam corecomprises providing a porous thermoplastic syntactic foam core having aplurality of dispersed microballoons.
 3. The method as recited in claim1 wherein the step of forming the foam core comprises preparing the foamcore within a mold substantially having the desired configuration. 4.The method as recited in claim 1 wherein the step of forming the foamcore comprises the steps of: preparing the foam core in a firstconfiguration; removing portions of the foam core from the firstconfiguration until the foam core is substantially formed into thedesired configuration.
 5. The method as recited in claim 1 wherein thestep of providing a foam sealing adhesive film comprises providing aprefabricated sheet of a syntactic foam sealing adhesive film having aplurality of glass microballoons intermixed with a second resin matrix,said prefabricated sheet being “B” stage cured.
 6. The method as recitedin claim 1 wherein the step of providing an outer skin comprises thesteps of: providing a sheet of a prepreg material; and forming the sheetof the prepreg material into at least a portion of the desiredconfiguration.
 7. The method as recited in claim 6, and furthercomprising the step of curing the formed prepreg material to form astructural outer skin.
 8. The method as recited in claim 6 wherein thestep of curing comprises curing the outer skin and the sealing adhesivefilm together.
 9. The method as recited in claim 8 wherein the step ofcuring comprises the steps of: placing the structural article into anautoclave; and heating the structural article to facilitate curing. 10.A method of fabricating a three dimensional structural article having adesired configuration from a plurality of low density materials, eachmaterial having a low radar reflectivity characteristic, said methodcomprising the steps of: providing a structural thermoplastic syntacticfoam core; forming said foam core into substantially the desiredconfiguration, said formed foam core having at least one exposedsurface; providing an outer skin material having a plurality ofstructural fibers intermixed with a resin matrix; forming said outerskin to conform to at least said exposed surface; curing said outer skinto form a structural outer skin configured to fit over and mate with theexposed surface of the foam core; providing a layer of a syntactic foamsealing adhesive film having a low dielectric constant and adapted forsealing the foam core and adhering with the outer skin; supporting thefoam core; placing the layer of sealing adhesive film over the exposedsurface on the foam core; placing the structural outer skin on the foamcore such that the layer of sealing adhesive film is sandwiched betweenthe structural outer skin and the exposed surface; and curing thestructural article, such that the sealing adhesive film secures theouter skin to the foam core.
 11. The method as recited in claim 10wherein said step of forming the foam core comprises the steps ofjoining a plurality of foam core sections to substantially form saiddesired configuration.
 12. The method as recited in claim 11 wherein thestep of curing plurality of foam core sections comprises the steps of:placing a layer of a second syntactic foam sealing adhesive film on aportion of a first foam core section; placing a second foam core sectionagainst the second sealing adhesive film such that the second sealingadhesive film is sandwiched between the first foam core section and thesecond foam core section; maintaining each of the form core sectionstogether; and curing the layer of the second syntactic foam sealingadhesive film to join the foam core sections.
 13. The method as recitedin claim 10 wherein the step of curing comprises the steps of: placingthe formed structural article into an autoclave; and heating the formedstructural article to a temperature of between approximately 300 degreeF. and 400 degrees F.
 14. The method as recited in claim 10 wherein thestep of providing a layer of a foam sealing adhesive film comprises thesteps of: providing a second resin matrix; heating the resin matrix;adding a plurality of microballoons to said resin matrix; mixing saidmicroballoons and said second resin matrix; forming the mixture into athin layer; and cooling the mixture.
 15. The method as recited in claim14 and further comprising the step of adding a plurality of silicaparticles to reduce the flow characteristics of the mixture prior toforming the mixture.
 16. The method as recited in claim 14 wherein thestep of forming the mixture into a thin layer comprises using a doctorblade to form the thin layer having a thickness of between 0.005 inchesand 0.03 inches.